Rectifiers have been used for many years to supply a structure with an impressed current to retard its corrosion. The impressed current creates a potential between the structure and the material surrounding it which opposes the electrolytic action which would otherwise accelerate the corrosive forces present in a mixed medium. This technique is generally referred to as cathodic protection.
The generally accepted parameter which is used to determine if sufficient cathodic protection current is being supplied is to measure the potential between the structure and a reference electrode, or reference cell. A lower or higher potential is usually indicative of a lower or higher amount of cathodic protection and is also directly related to a lower or higher amount of cathodic protection current being delivered by a rectifier or the like. A serious problem in measuring or monitoring the potential between the structure and the reference cell is that when the rectifier is delivering current, there is an associated IR drop due to the surrounding material spaced between the cathode created at the structure and the reference half cell. As the amount of IR drop is directly dependent upon the properties of the electrolyte and the distance between the cathode and referene half cell, it is virtually impossible to measure with any accuracy the impressed potential, and hence the amount of protection, while the rectifier is delivering current to the structure.
In the prior art, this problem has been addressed by various schemes such as those disclosed in U.S. Pat. Nos. 3,634,222 and 4,080,272. The first of these U.S. Pat. No. 3,634,222, discloses and claims a circuit which is particularly adapted for use with an SCR type rectifier in which the anode current is interrupted or turned off for a brief amount of time. During a first portion of this off time, the current is allowed to decay and the sampling takes place in a second portion of the off time. The SCR is then turned back on and rectifier output is resumed. In U.S. Pat. No. 4,080,272, the operation of the rectifier is not interrupted. Instead, the rectifier is designed so that its output voltage crosses through zero every half cycle with the sampling taking place during this zero voltage condition. As can be appreciated, neither of these schemes can be used with a rectifier having output chokes or efficiency filters which smooth the output waveform and which maximize the conversion efficiency of the rectifier. This is because both of these schemes rely upon the rapid decay to zero or the periodic crossing through zero voltage in the rectifier output. With a choke or efficiency filter, decay is greatly retarded (intentionally) which would prevent either of these methods from being effective in measuring the IR drop free minimum potential between the structure and a reference half cell.
Another disadvantage with the methods described in both of these prior art patents is that the sample of potential is taken relatively soon after the output current reaches a zero value. The actual minimum potential may actually occur at a time just before the rectifier output is re-established, or later in the sampling time period. Furthermore, neither of these references describes a circuit which "tracks" the minimum potential measured, and instead suggests that this sampling potential is a singular value which can be measured and stored at a particular point in time. As the system is a dynamic system, with currents and voltages decaying and being re-established on a periodic basis, it is impossible that this is the case. Instead, it is more probable that the minimum potential could occur for only a short period of time during the sampling time and at a different point in time for different output currents. However, with the circuits of the prior art, there is no means to discriminate between any of the different values which might be available during the sampling period.
Another problem in the prior art has been to combine one of these patented methods of measuring IR drop free minimum reference cell potential with a rectifier suitable for use under conditions requiring low voltage and high current output. For example, in a bridge type rectifier using at least two blocking diodes in the bridge, the SCRs are fired on for only a short period of time to deliver the required low output voltage, and the blocking diodes prevent the flow of current through the transformer at those times other than when the voltage across the transformer is positive. Thus, the output current must flow during a relatively low percentage of time with the output being zero during a large portion of each half cycle. The use of an output choke with a four SCR bridge circuit would permit positive output current under negative transformer voltage conditions although this is a much more expensive design requiring much more complicated control circuitry to achieve the firing of all of the SCRs. Furthermore the output current does not necessarily go to zero which makes this configuration unsuitable for use with the sampling method disclosed in U.S. Pat. No. 4,080,272.
A circuit which has been used in these low voltage, high current applications has a center tap transformer with two SCRs in each of the legs and a choke in one of the output lines. The action of the choke and the elimination of the blocking diodes permits the flow of current through the transformer and into the load even under negative voltage conditions. Thus, with the center tap, two SCR, choke type design a lower RMS current is experienced as current is conducted for a longer period of time in each cycle. This results in better operation, and the ability to use lower capacity components for any particular application over that required in the bridge type design having two blocking diodes. Unfortunately, neither method of the prior art for detecting the minimum reference cell potential can be used with this center tap, two SCR, choke type design as the output current never falls to zero. Thus, there is no sampling time available for measuring the minimum reference cell potential and merely turning off the SCrs does not achieve zero current output because of the action of the choke.
To solve these and other problems in the prior art, applicant has succeeded in designing and developing a rectifier control which is suitable for used with a center tap, two SCR, choke design and which measures the IR drop free minimum reference cell potential at its lowest value during its sampling period and automatically adjusts the output of the rectifier to achieve the desired cathodic protection potential. As applicant's design continuously monitors the reference cell potential, if the rectifier output falls to zero during normal operation the minimum reference cell potential is picked up and used to modify the phase angle at which the SCRs are fired. Thus, applicant's design is the first design which continuously monitors the minimum reference cell potential and is not limited to a defined portion of the waveform which could result in incorrectly higher readings and a lower protective current delivered to the structure than is desired.
Applicant's design generally includes a shunting transistor circuit which is periodically pulsed on to shunt or electrically connect the output lines of the rectifier and thus force the voltage and current delivered to the structure to zero. The reference cell potential is input to a minimum detector circuit which continuously monitors it but which holds the minimum value which exists as a result of the diversion or interruption of the output current. This minimum reference cell potential is then compared to a voltage corresponding to the desired reference cell potential and the amplified difference represents an error signal which is compared to a ramp voltage and used to generate a firing voltage for the SCRs. By using a shunting transistor, a choke or other impedance may be provided in the output of the rectifier which maintains the current through the transformer and permits sharp switching of the output. As the SCRs themselves are not used to create the zero output condition, use of the choke is feasible as well as efficiency filters which greatly increase the conversion efficiency and effectiveness of the rectifier itself. Of course, as applicant's device independently creates a "zero" output condition, it may be used with three phase rectifiers as well as single phase rectifiers unlike the method described in U.S. Pat. No. 4,080,272 which is limited to single phase circuits as it requires a zero crossing point in the rectifier output itself.
These and other advantages may be more fully appreciated by referring to the drawings and the description of the preferred embodiment which follows .